This is the first application filed for the present invention.
Not applicable.
The invention relates to spread spectrum communications and in particular to methods and apparatus for providing frame quality indication in decoding received coded spread spectrum signals.
Multiple access modulation techniques are some of the most efficient techniques for utilizing the limited wireless frequency spectrum. Examples of such techniques include time division multiple access (TDMA), frequency division multiple access (FDMA), and code division multiple access (CDMA).
CDMA modulation employs a spread spectrum technique for the transmission of information. A spread spectrum communications system uses a modulation technique that spreads a transmitted signal over a wide frequency band, this frequency band is substantially wider than the minimum bandwidth required to transmit the signal. The transmission signal is spread over the wide frequency band according to a spreading code associated with an intended receiver.
In a spread spectrum communications system, multiple spread spectrum signals are transmitted simultaneously in the same frequency band. A particular spread spectrum receiver determines which signal is intended for that particular receiver using a unique spreading code associated with the particular receiver. The spread spectrum signals in the frequency band not associated with the particular spreading code of the particular receiver appear as background noise to that receiver. The intended signal is to be discerned from the background noise.
Although spread spectrum transmission techniques provide excellent multiplexing transmission efficiencies compared to other signal transmission techniques, an induced signal degradation is experienced by a particular spread spectrum signal due to noise created by the other simultaneously transmitted spread spectrum signals. This is a limiting factor affecting the multiplexing efficiency of spread spectrum communications systems. This noise can be characterized as white noise and can be modeled using a Gaussian noise model referred to as additive white Gaussian noise (AWGN).
One of the major technical challenges in communicating effectively and reliably over wireless links is overcoming signal interference. There are may types of signal interference, each of which affects a spread spectrum signal differently. One type of interference affects the amplitude of the spread spectrum signal as it is received at a receiver. Another type of interference affects the phase of the spread spectrum signal as it is received at the receiver. Yet another type of interference affects the ability of the receiver to discern the signal from background noise such as thermal noise and other radio frequency pick-up. This last type of noise interference can be modeled as additive white Gaussian noise applied to the spread spectrum signal.
In general, noise, be it due to other spread spectrum signal transmissions in the frequency band or noise interference, reduces the signal-to-noise ratio (SNR) of a received spread spectrum signal. A reduced SNR makes it difficult to demodulate a particular spread spectrum signal intended for the receiver. This has a negative impact on multiplexing efficiency.
Considering that increasing multiplexing efficiency is a desired goal in spread spectrum communications, it is therefore of a competitive advantage to determine the reliability of wireless links employing spread spectrum communications techniques. Typically a transmitted signal employing spread spectrum techniques has a transmission structure: the payload is digital, and the stream of data conveyed between a spread spectrum transmitter and a spread spectrum receiver is divided into frames. The frames have an associated frame transmission rate and an associated frame transmission time. A frame quality indication is used to specify the reliability of reception of frames at the receiver.
Another spread spectrum communications area in which the reliability of a spread spectrum communications link is important, is the control of a soft handover of an established spread spectrum communications session. Typical mobile telephony implementations use mobile telephones with limited reception capabilities to exchange frames over a spread spectrum frequency bandwidth with a base station within a limited distance from the base station. Typically the distance is limited to one over which the amplitude of the spread spectrum signal diminishes to a value which makes the spread spectrum signal indistinguishable from background noise for the purposes of demodulation/decoding. Given this arrangement, such a spread spectrum communications session is said to take place within a cell. Therefore, as a mobile telephone nears a border of the cell, the mobile telephone experiences a degradation in the reception of the spread spectrum signal equivalent to a contamination by white noise. Consequently, determination of frame quality can also be used to initiate soft handover of spread spectrum communications sessions. Smooth soft handovers are essential to a mobile telephone end user""s experience.
Techniques for assessing the reliability of wireless links employing spread spectrum transmission techniques have been proposed. These techniques address a variety of sources of interference and have yielded varied levels of success.
U.S. Pat. No. 5,802,105 which issued on Sep. 1, 1998 to Tiedemann, Jr. et al. describes a system which transmits a test sequence of digital data over a wireless communications channel established between a transmitter and a receiver. The accuracy of transmission over the wireless communications channel is determined by comparing the received test sequence to a replica test sequence generated at the receiver. While the teachings of Tiedemann, Jr. et al. have merit, in implementing these teachings a considerable fraction of a transmitted payload stream is dedicated to determining the accuracy of transmission. A considerable amount of computation is also required at the receiver to enable determination of the accuracy of transmission. Besides, the accuracy of the indicator is dependent on an overall accuracy of transmission.
U.S. Pat. No. 5,790,596 which issued Aug. 4, 1998 to Sexton describes a system which estimates a slowly changing channel parameter such as channel gain and phase shift to improve receiver performance for subsequent transmissions. Sexton teaches the use of frame quality indicating information transmitted as part of the transmission signal. The frame quality indicating information is processed subsequent to demultiplexing the transmitted signal and used to selectively accept data frames. Rejected data frames are discarded. Sexton also teaches the use of an encoder block in the receiver to convolute and interleave a de-interleaved, de-convolved and demultiplexed accepted data frame to determine a degree of degradation of the transmitted signal with respect to channel gain or phase distortions. A portion of the re-encoded frame is compared with the received frame to compute the slowly changing channel parameter. The computed channel parameter is fed back to the receiver to improve reception of future transmissions on the theory that the channel parameter will remain substantially constant for a period of at least two received, accepted frames. While the teachings of Sexton have merit, frame acceptance is dependent solely on the transmitted frame quality indicator bits. This contributes to overhead and may result in the rejection of good frames. A considerable amount of computation is also required at the receiver in processing the frame quality indicating information in addition to computing the channel parameter.
Therefore there is a need for methods and apparatus for providing an efficient frame quality determination for a received spread spectrum signal subjected to noise induced distortions which does not rely on an overhead of sacrificial frame quality information in the transmitted data frames.
It is an object of the invention to provide a method of assessing the frame quality of a received wireless signal and improving multiplexing efficiency by eliminating a need for transmitting frame quality information along with the payload.
According to an aspect of the invention, a method of indicating the frame quality of a received frame of a received wireless signal is provided. The method comprises five steps. The received frame is demodulated using a demodulator and the demodulator provides a demodulated frame. The demodulated frame is decoded using a decoder and the decoder provides a decoded frame. After decoding, the decoded frame is encoded using an encoder matched to the decoder and the encoder provides a re-encoded frame. The re-encoded frame is modulated by a modulator matched to the demodulator and the modulator provides a re-modulated frame. The frame quality indication is computed based on a comparison between the received frame and the re-modulated frame.
According to another aspect of the invention, a receiver adapted to indicate the frame quality of a received frame of a received wireless signal is provided. The receiver comprises: a demodulator adapted to demodulate a received frame and provide a demodulated frame, a decoder adapted to decode the demodulated frame and provide a decoded frame, an encoder matched to the decoder, the encoder being adapted to encode the decoded frame and provide a re-encoded frame, a modulator matched to the demodulator, the modulator being adapted to modulate the re-encoded framed and provide a modulated signal, a correlator adapted to correlate the received frame with the re-modulated frame providing a correlation level and a decision circuit adapted to compute the frame quality indication of the received frame based on the correlation level.
According to another aspect of the invention, a frame quality indication generator is provided. The frame quality indication generator is adapted to process a received frame of an encoded wireless signal and a decoded frame resulting from the encoded wireless signal being demodulated by a demodulator and decoded by a decoder. The frame quality indication generator comprises: an encoder matched to the decoder, the encoder being adapted to encode the decoded frame and provide a re-encoded frame, a modulator matched to the demodulator, the modulator being adapted to modulate the re-encoded frame and provide a re-modulated frame, a correlator adapted to correlate the received frame with the re-modulated frame and provide a correlation level, and a decision circuit adapted to compute the frame quality indication of the received frame based on the correlation level.
According to another aspect of the invention, a method of initiating soft handover of a wireless communication session between base stations is provided. The method uses a frame quality indication to selectively initiate a soft handover.
According to another aspect of the invention, a method of decoding a received wireless signal is provided. The method uses a frame quality indication to selectively accept decoded frames.